Photovoltaic structures having multiple absorber layers separated by a diffusion barrier

ABSTRACT

Photovoltaic structures having multiple absorber layers separated by a diffusion barrier are provided. In one aspect, a method of forming an absorber on a substrate includes: depositing a first layer of light absorbing material on the substrate; depositing a diffusion barrier; depositing a second layer of light absorbing material on the diffusion barrier, wherein the first layer of light absorbing material has a different band gap from the second layer of light absorbing material; and annealing the absorber, wherein the diffusion barrier prevents diffusion of elements between the first layer of light absorbing material and the second layer of light absorbing material during the annealing. A solar cell and method for formation thereof are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/409,142 filed on Jan. 18, 2017, now U.S. Pat. No. 10,361,331, thecontents of which are incorporated by reference herein.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Contract numberDE-EE0006334 awarded by Department of Energy. The Government has certainrights in this invention.

FIELD OF THE INVENTION

The present invention relates to photovoltaic structures and moreparticularly, to photovoltaic structures involving multiple absorberlayers separated by a diffusion barrier.

BACKGROUND OF THE INVENTION

Materials utilized for photovoltaic absorbers have become increasinglycomplex as those presently available are approaching their efficiencylimits. In order to increase the efficiency of multi-elementalphotovoltaic absorbers, the material is often graded in composition sothat the energy gap (band gap) varies from the front to the back of theabsorber in order to drive the electrons and holes more effectivelytoward their respective electrical contacts, and also to maximizeabsorption of sunlight.

In many materials, this compositional and/or elemental grading cannot beachieved because rapid diffusion of the elements in the absorber occursduring thermal processing. This problem affects not only photovoltaicdevices but is a widespread problem in material science, since hightemperature processing diffusively drives elements to undesired places.

Therefore, improved techniques for forming multi-elemental photovoltaicabsorbers would be desirable.

SUMMARY OF THE INVENTION

The present invention provides photovoltaic structures having multipleabsorber layers separated by a diffusion barrier. In one aspect of theinvention, a method of forming an absorber on a substrate is provided.The method includes: depositing a first layer of light absorbingmaterial on the substrate; depositing a diffusion barrier on the firstlayer of light absorbing material; depositing a second layer of lightabsorbing material on the diffusion barrier to form a stack of layers oflight absorbing materials on the substrate, wherein the first layer oflight absorbing material has a different band gap from the second layerof light absorbing material, and wherein the stack of layers of lightabsorbing materials form the absorber on the substrate; and annealingthe absorber, wherein the diffusion barrier prevents diffusion ofelements between the first layer of light absorbing material and thesecond layer of light absorbing material during the annealing.

In another aspect of the invention, a method of forming a solar cell isprovided. The method includes: coating a substrate with a layer of aconductive material; forming an absorber on the layer of conductivematerial by i) depositing a first layer of light absorbing material onthe layer of conductive material, ii) depositing a diffusion barrier onthe first layer of light absorbing material, iii) depositing a secondlayer of light absorbing material on the diffusion barrier to form astack of layers of light absorbing materials on the layer of conductivematerial, wherein the first layer of light absorbing material has adifferent band gap from the second layer of light absorbing material,iv) annealing the absorber, wherein the diffusion barrier preventsdiffusion of elements between the first layer of light absorbingmaterial and the second layer of light absorbing material during theannealing; forming a buffer layer on the absorber; forming a transparentfront contact on the buffer layer; and forming a metal grid on thetransparent front contact.

In yet another aspect of the invention, a solar cell is provided. Thesolar cell includes: a substrate; a layer of a conductive material onthe substrate; an absorber on the layer of conductive material, theabsorber having: i) a first layer of light absorbing material on thelayer of conductive material, ii) a diffusion barrier on the first layerof light absorbing material, iii) a second layer of light absorbingmaterial on the diffusion barrier to form a stack of layers of lightabsorbing materials on the layer of conductive material, wherein thesecond layer of light absorbing material has a higher band gap than thefirst layer of light absorbing material; a buffer layer on the absorber;a transparent front contact on the buffer layer; and a metal grid on thetransparent front contact.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary methodology forforming a multi-elemental photovoltaic absorber according to anembodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating a substrate coated witha layer of an electrically conductive material according to anembodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a multi-elementalabsorber having been formed on the electrically conductive material, themulti-elemental absorber having at least two layers of light absorbingmaterials separated by a diffusion barrier according to an embodiment ofthe present invention;

FIG. 4 is a cross-sectional diagram illustrating a buffer layer havingbeen formed on the absorber according to an embodiment of the presentinvention;

FIG. 5 is a cross-sectional diagram illustrating a transparent frontcontact having been formed on the buffer layer according to anembodiment of the present invention;

FIG. 6 is a cross-sectional diagram illustrating a metal grid havingbeen formed on the transparent front contact according to an embodimentof the present invention;

FIG. 7 is a cross-sectional diagram illustrating an exemplary solar cellformed using the present techniques wherein band gap grading is achievedby varying a concentration of sulfur according to an embodiment of thepresent invention;

FIG. 8 is a cross-sectional diagram illustrating another exemplary solarcell formed using the present techniques wherein band gap grading isachieved by using different light absorbing materials according to anembodiment of the present invention;

FIG. 9 is a cross-sectional diagram illustrating yet another exemplarysolar cell formed using the present techniques wherein at least oneadditional layer of light absorbing material is included in the absorberstack according to an embodiment of the present invention;

FIG. 10 is a cross-sectional diagram illustrating still yet anotherexemplary solar cell formed using the present techniques wherein bandgap grading is achieved by varying a concentration of sulfur and byusing different light absorbing materials according to an embodiment ofthe present invention;

FIG. 11 is a diagram illustrating the effectiveness of graphene as adiffusion barrier in an absorber prepared according to the presenttechniques according to an embodiment of the present invention; and

FIG. 12 is a diagram illustrating current-voltage (I/V) characteristicsof a solar cell prepared according to the present techniques accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques that utilize the interspersing ofgraphene between layers of materials with different elementalconcentrations or different elements in multi-elemental photovoltaicabsorbers and other functional materials. In one exemplary embodimentdescribed below, a photovoltaic device is formed by covering a (e.g.,molybdenum (Mo)-coated glass) substrate with a layer of a lightabsorbing material such as CZT(S,Se). This layer of light absorbingmaterial is then covered with one or more layers of a diffusion barriersuch as graphene, followed by the deposition of a second layer of lightabsorbing material having a different elemental concentration (e.g., adifferent sulfur (S) to selenium (Se) ratio) and/or a differentelemental composition (e.g., AZT(S,Se)) than the first, and so on,forming a stack of layers of the light absorbing material.

By way of example only, the light absorbing layers separated by thediffusion barrier may have varying band gaps. The multiple layers withvarying band gaps form a “graded band gap” structure that moreeffectively absorbs light and drives the photo-generated electrons andholes more efficiently to their respective electrical contacts. Thediffusion barrier prevents intermixing of the various light absorbinglayers during high temperature annealing used to crystallize the film.

An overview of the present techniques for forming a multi-elementalphotovoltaic absorber (or simply “absorber”) on a substrate is nowprovided by way of reference to methodology 100 of FIG. 1. In step 102,a layer of a first light absorbing material (Light Absorbing Material 1)is deposited on the substrate. According to an exemplary embodiment, thepresent techniques are implemented in the fabrication of a solar cell.Thus, the substrate in that example would be a suitable solar cellsubstrate (such as soda lime glass (SLG)) coated with a layer(s) of aconductive material (such as Mo). A variety of suitable solar cellsubstrate and conductive materials are provided below.

According to one exemplary embodiment, the first light absorbingmaterial contains S and/or Se at a given (S:Se) ratio. Suitable lightabsorbing materials include, but are not limited to CZT(S,Se) and/orAZT(S,Se) materials. As its name implies, CZT(S,Se) includes copper(Cu), zinc (Zn), tin (Sn), and at least one of S and Se. Similarly,AZT(S,Se) includes silver (Ag), Zn, Sn, and at least one of S and Se.

The band gap of CZT(S,Se) and AZT(S,Se) can be varied by varying the Sto Se ratio in the material, with an increase in the concentration of Scorrelating with an increase in the band gap. Thus, according to anexemplary embodiment, the concentration of S is increased in eachsuccessive layer of light absorbing material to end up with the lightabsorbing layer having the greatest concentration of S and greatest bandgap at the top of the stack. See below. Thus to use a non-limitingexample to illustrate this concept, if each of the light absorbinglayers that makes up the absorber is CZT(S,Se), then the S:Se ratio inthe layer of the first layer of light absorbing material can beconfigured (i.e., during formation) to be less than the S:Se ratio inthe next highest (i.e., second) layer of light absorbing material (seestep 106 described below), and so on. For instance, the first layer oflight absorbing material might be configured to contain no S (only Se),while the second layer of light absorbing material will contain acombination of S and Se, or only S (no Se). As a result, the first layerof light absorbing material will have a smaller band gap than the secondlayer of light absorbing material. The same concept applies to AZT(S,Se)materials.

The band gap of the absorber can also be varied by varying thecomposition of the light absorbing material. For instance, AZT(S,Se) hasa greater band gap than CZTSe, having only Se (no S) (e.g., 1.33electron volts (eV) for AZT(S,Se) versus 1.0 eV for CZTSe). Thus, toachieve the above-described absorber configuration (i.e., having thelight absorbing layer with the greatest band gap at the top of thestack), the layer(s) of the light absorbing material lower in theabsorber stack can include CZTSe while the layer(s) of the lightabsorbing material higher in the absorber stack can include AZT(S,Se).

Combinations of the above-described mechanisms (i.e., varying S:Se ratioand varying absorber material composition) are also contemplated herein.For example, in the lower layers of the absorber stack a CZT(S,Se) lightabsorbing material can be employed with increasing S concentrationmoving up the stack, while in the upper layer(s) of the absorber stack aAZT(S,Se) light absorbing material can be employed. This configurationwill be described in detail below.

In accordance with the present techniques, the layers of light absorbingmaterials may be deposited using vacuum-based, solution-based, or othersuitable approaches. See for example U.S. Patent Application PublicationNumber 2012/0061790 filed by Ahmed et al., entitled “Structure andMethod of Fabricating a CZTS Photovoltaic Device by Electrodeposition,”the contents of which are incorporated by reference as if fully setforth herein. Suitable solution-based Kesterite fabrication techniquesare described for example in U.S. Patent Application Publication Number2013/0037111 filed by Mitzi et al., entitled “Process for Preparation ofElemental Chalcogen Solutions and Method of Employing Said Solutions inPreparation of Kesterite Films,” the contents of which are incorporatedby reference as if fully set forth herein. A suitable particle-basedprecursor approach for CZT(S,Se) formation is described for example inU.S. Patent Application Publication Number 2013/0037110, filed by Mitziet al., entitled “Particle-Based Precursor Formation Method andPhotovoltaic Device Thereof,” the contents of which are incorporated byreference as if fully set forth herein. Using these processes, thecontents of each layer of the light absorbing material can be tuned bycontrolling the composition during deposition and/or post-depositionduring annealing of the deposited film which can be performed in thepresence of excess chalcogen—see below.

Co-evaporation of the constituent components from their respectivesources may also be employed. See, for example, U.S. patent applicationSer. No. 14/936,131 by Gershon et al., entitled “Photovoltaic DeviceBased on Ag₂ZnSn(S,Se)₄ Absorber,” the contents of which areincorporated by reference as if fully set forth herein. Valve-controlledsources of S and Se can be implemented to control the S to Se ratio, ifso desired. See, for example, U.S. Patent Application Publication Number2012/0100663 by Bojarczuk et al., entitled “Fabrication of CuZnSn(S,Se)Thin Film Solar Cell with Valve Controlled S and Se,” the contents ofwhich are incorporated by reference as if fully set forth herein.

Optionally, after the first layer of light absorbing material has beenformed on the substrate, an intermediate anneal may be performed in anenvironment containing excess chalcogen (e.g., S and/or Se). See step103. Elements such as S and Se are volatile, and are lost from the filmduring heating. Thus, providing excess S and/or Se serves to replacethese elements. Annealing also improves grain characteristics of thematerial. Further, as provided above, the ratio of S:Se can be variedthroughout the layers of light absorbing material to control the bandgap. Performing this intermediate anneal in a chalcogen-containingambient provides a convenient mechanism to regulate the S and/or Seconcentration. For instance, to successively increase the Sconcentration in each layer deposited on the stack, an intermediateanneal can be performed after the deposition of each layer. The amountof S in the ambient employed during the intermediate anneal can beincreased for each layer, thereby increasing the S concentration (andband gap) of each successive layer formed on the stack. The diffusionbarrier placed after each layer retains the elements in the presentlayer by preventing them from diffusing down into lower layers beneaththe diffusion barrier. According to an exemplary embodiment, theintermediate anneal is performed at a temperature of from about 400degrees Celsius (° C.) to about 800° C., and ranges therebetween, for aduration of from about 100 seconds to about 120 seconds, and rangestherebetween. The use of intermediate anneals in a chalcogen environmentto tune the band gap are, however, optional such as in the case wherethe compositions of the various layers of light absorbing materials aretuned during deposition of the material. In that case, a single anneal(see step 112—described below) can be performed after completion of thestack to improve grain characteristics.

Next, in step 104 a diffusion barrier is formed on the first layer oflight absorbing material. Suitable diffusion barrier materials include,but are not limited to, graphene, titanium nitride (TiN), tantalumnitride (TaN), and tungsten nitride (WN). According to an exemplaryembodiment, the diffusion barrier includes graphene. Graphene istransparent to light and thus is a suitable choice for use in a (light)absorber of a solar cell. For example, at least one layer of graphene isdeposited on the first layer of light absorbing material in step 104.Graphene can be transferred from a substrate (on which the graphene hasbeen grown) to the first layer of light absorbing material using, e.g.,an exfoliation process. In this example, the diffusion barrier caninclude a single layer (i.e., one atomic layer of graphene) or multiplelayers (i.e., from about 1 layer to about 5 layers of graphene, andranges therebetween). With these few layers (i.e., about 5 or less) ofthe graphene, the diffusion barrier will still remain transparent, andin production it may be challenging to apply only a single layer.Graphene is also conducting and will not decompose during hightemperature anneal steps required to produce high efficiency CZT(S,Se).

In step 106, a second layer of a light absorbing material (LightAbsorbing Material 2) is deposited on top of the diffusion barrier,forming a stack of light absorbing layers on the substrate. This stackof layers collectively serves as an absorber.

In order to achieve band gap grading, the second layer of lightabsorbing material differs in concentration and/or composition from thefirst layer of light absorbing material. Namely, as described above, theconcentration of sulfur (S) can be increased in the second layer of thelight absorbing material as compared to the first layer of lightabsorbing material, which increases the band gap of the second layer oflight absorbing material as compared to the first layer of lightabsorbing material. Compositionally, the second layer of light absorbingmaterial can be a different (greater band gap) material than the firstlayer of light absorbing material. For instance, the first layer oflight absorbing material can be CZT(S,Se), while the second layer oflight absorbing material can be AZT(S,Se). As described above, AZT(S,Se)has a greater band gap than CZTSe.

As above, an intermediate anneal in a chalcogen ambient can again beperformed after the second layer of light absorbing material to tune theS:Se ratio in the second layer of light absorbing material. See step107. Suitable conditions for this intermediate anneal were providedabove. To increase the band gap, one could simply increase the amount ofS provided during this (subsequent) intermediate anneal to increase theamount of S in the second layer of light absorbing material as comparedto the first layer of light absorbing material. The following exampleillustrates this concept. After deposition of the first layer of lightabsorbing an intermediate anneal is performed in a first chalcogencontaining environment. The idea is to start off the stack with thelowest S-concentration, so the first chalcogen containing environmentpreferably contains more Se than S, or perhaps no S at all. Followingdeposition of the barrier layer and the second layer of light absorbingmaterial, another intermediate anneal is performed in a secondchalcogen-containing environment having a greater S concentration thanthe first chalcogen-containing environment. For instance, the secondchalcogen-containing environment contains more S than Se, or perhaps noSe at all. That way, the concentration of S (and thus the band gap)increases in the layers moving up the stack. The diffusion barrier(formed in step 104) prevents intermixing of elements between thelayers.

The stack can be complete with two layers of light absorbing material oroptionally, as shown in FIG. 1, an additional diffusion barrier can bedeposited on the second layer of light absorbing material (step 108) andan additional layer of light absorbing material can be deposited on theadditional diffusion barrier (step 110). The barrier layers arepreferably the same throughout the stack. The additional light absorbinglayer is configured to differ in composition and/or concentration toincrease the band gap in the same manner as described above. Further, asshown in FIG. 1, steps 108 and 110 can be repeated x times until thedesired number of light absorbing layers is present in the stack. Thus,each time a light absorbing layer is deposited it is topped with adiffusion barrier before the next light absorbing layer is placed. Byway of example only, multiple iterations of steps 108 and 110 can beperformed to place multiple layers of CZT(S,Se) or multiple layers ofAZT(S,Se), wherein each layer added to the stack has a higher Sconcentration than the previous layer, and hence a greater band gap.

Once the desired number of light absorbing layers have been formed inthe stack, a final anneal of the absorber is performed in step 112.Annealing serves to improve the grain structure of the film. Accordingto an exemplary embodiment, the anneal is performed at a temperature offrom about 400° C. to about 800° C., and ranges therebetween, for aduration of from about 100 seconds to about 120 seconds, and rangestherebetween. Preferably, the annealing is performed in an environmentcontaining excess chalcogen (e.g., S and/or Se) which, as describedabove, serves to replace these volatile elements lost during heatingand/or can be used to tune the band gap. When anneals in a chalcogenenvironment are used to tune the band gap, then the anneal performed instep 112 serves the same purpose as the intermediate anneals (describedabove), except that it serves to tune the band gap of the last (top)layer of the stack. Advantageously, the barrier layer(s) preventinter-diffusion of elements between the layers of light absorbingmaterials in the stack.

According to an exemplary embodiment, the present techniques areimplemented in the fabrication of a solar cell. This exemplaryembodiment is now described by way of reference to FIGS. 2-6. As shownin FIG. 2, the process begins with a substrate 202 coated with a layer204 (or optionally multiple layers represented generically by layer 204)of an electrically conductive material.

Suitable solar cell substrates 202 include, but are not limited to,glass (e.g., soda lime glass (SLG)), ceramic, metal foil, or plasticsubstrates. Suitable materials for forming the electrically conductivelayer 204 include, but are not limited to, Mo, molybdenum trioxide(MoO₃), gold (Au), nickel (Ni), tantalum (Ta), tungsten (W), aluminum(Al), platinum (Pt), titanium nitride (TiN), silicon nitride (SiN), andcombinations thereof (such as an alloy of one or more of these metals oras a stack of multiple layers such as MoO₃+Au). The electricallyconductive layer 204 will serve as a back contact of the solar cell.

According to an exemplary embodiment, the conductive layer 204 forms acoating on the substrate 202 having a thickness of greater than about0.1 micrometers (μm), for example, from about 0.1 μm to about 2.5 μm,and ranges therebetween. The various layers of the solar cell will bedeposited sequentially using a combination of vacuum-based and/orsolution-based approaches. For instance, according to an exemplaryembodiment, the electrically conductive material 204 can be depositedonto the substrate 202 using a process such as evaporation orsputtering.

As shown in FIG. 3, an absorber 302 is next formed on the conductivelayer 204. Absorber 302 is formed using the process described inconjunction with the description of FIG. 1, above. For instance,absorber 302 is shown in this example to include a first layer of alight absorbing material (Light Absorbing Material 1) and a second layerof a light absorbing material (Light Absorbing Material 2) separated bya diffusion barrier. Preferably, the second layer of a light absorbingmaterial has a greater band gap than the first layer of a lightabsorbing material. As described above, this can be accomplished in anumber of different ways. One way is to increase the S concentration inthe second layer. Another way is to use a higher band gap material inthe second layer, i.e., a CZT(S,Se) bottom layer and AZT(S,Se) toplayer. Further, it is notable that while only two layers of lightabsorbing materials are shown in this example, as provided above, morelayers can be added to the stack if so desired. An exemplary absorberstack configuration with more than two light absorbing layers isdescribed below. Suitable diffusion barrier materials were providedabove. Here the diffusion barrier is shown as a single layer, but it isto be understood that in practice the diffusion barrier can includemultiple layers, such as multiple layers of graphene.

Intermediate anneals after the deposition of individual layers of lightabsorbing material and/or a final anneal after the final layer of lightabsorbing material is formed on the stack can be performed to enhancethe grain structure of the absorber 302 and/or tune the band gap. Asdescribed above, these anneals can be performed in a chalcogen (e.g., Sand/or Se) environment. As provided above, suitable annealing conditionsinclude a temperature of from about 400 degrees ° C. to about 800° C.,and ranges therebetween, for a duration of from about 100 seconds toabout 120 seconds, and ranges therebetween.

As shown in FIG. 4, a buffer layer 402 is then formed on the absorber302. The buffer layer 402 forms a p-n junction with the absorber 302.According to an exemplary embodiment, the buffer layer has a thicknessof from about 100 angstroms (Å) to about 1,000 Å, and rangestherebetween.

Suitable materials for the buffer layer 402 include, but are not limitedto, cadmium sulfide (CdS), a cadmium-zinc-sulfur material of the formulaCd_(1-x)Zn_(x)S (wherein 0<x≤1), indium sulfide (In₂S₃), zinc oxide,zinc oxysulfide (e.g., a Zn(O,S) or Zn(O,S,OH) material), and/oraluminum oxide (Al₂O₃). According to an exemplary embodiment, the bufferlayer 402 is deposited on the absorber 302 using standard chemical bathdeposition.

A transparent front contact 502 is then formed on the buffer layer 402.See FIG. 5. Suitable materials for the transparent front contact 502include, but are not limited to, transparent conductive oxides (TCOs)such as indium-tin-oxide (ITO) and/or aluminum (Al)-doped zinc oxide(ZnO) (AZO). According to an exemplary embodiment, the transparent frontcontact 502 is formed on the buffer layer 402 by a process such assputtering.

As shown in FIG. 6, a metal grid 602 is then formed on the transparentfront contact 502. Suitable materials for forming the metal grid 602include, but are not limited to, nickel (Ni) and/or aluminum (Al).According to an exemplary embodiment, the metal grid 602 is formed onthe transparent front contact 502 using a process such as evaporation orsputtering.

A variety of different absorber configurations have been describedherein to achieve band gap grading. Some exemplary, non-limitingexamples are now provided. In the following examples, the same generalsolar cell as above is used with variations in the configuration of theabsorber. Thus, in the examples, like structures with theabove-described solar cell are numbered alike. In each of the examples,the band gap of the light absorbing layers in the absorber stackincreases incrementally as one moves up the stack. So, for instance, theband gap of the first layer of light absorbing material in the stack isless than the band gap of the next highest layer of light absorbingmaterial in the stack, and so on. As described above, this band gapgrading can be accomplished in a number of different ways. One way is toincrease the S concentration in the second layer. Another way is to usea higher band gap material in the second layer, i.e., a CZT(S,Se) bottomlayer and AZT(S,Se) top layer. Advantageously, the use of a diffusionbarrier between each light absorbing layer prevents inter-diffusion ofthe elements between the layers in the stack during anneal, thuspermitting effective band gap grading to be achieved.

In a first example shown in FIG. 7, the concentration of S is used totune the band gap of the absorber 302. For instance, the first layer oflight absorbing material (Light Absorbing Material 1) will have a lowerconcentration of S than the second layer of light absorbing material(Light Absorbing Material 2). CZT(S,Se) is used as the light absorbingmaterial in this example. Specifically, in this particular example, thefirst layer of light absorbing material (Light Absorbing Material 1)contains Se but no S. Accordingly, the abbreviation CZTSe is used toindicate that the first layer of light absorbing material (LightAbsorbing Material 1) contains no S. With Se (and no S), the first layerof light absorbing material (Light Absorbing Material 1) will have aband gap (E_(g)) of about 1.0 eV. Adding S increases the band gap. Thus,with a second layer of light absorbing material (Light AbsorbingMaterial 2) containing both S and Se (hence CZT(S,Se)), the band gap ofthe second layer of light absorbing material (Light Absorbing Material2) has a higher band gap (than the first layer) of from about 1.2 eV toabout 1.4 eV, and ranges therebetween. Alternatively, the second layerof light absorbing material (Light Absorbing Material 2) could insteadbe configured to contain S but no Se (which is abbreviated herein asCZTS) which would further increase the band gap of the second layer oflight absorbing material (Light Absorbing Material 2) to about 1.45 eV.To do so, however, it might be desirable to include an intermediatelayer containing both S and Se (i.e., CZT(S,Se)) to achieve a smootherband gap gradient. See, for example, FIG. 9—described below. The samegeneral principle applies to an AZT(S,Se) material. for instance, anAZT(S,Se) light absorbing material can have a band gap ranging from 1.3eV to 2.01 eV depending on the S concentration, wherein a band gap of2.01 eV is achievable using S (but no Se), i.e., AZTS. See, for example,Yuan et al., “Engineering Solar Cell Absorbers by Exploring the BandAlignment and Defect Disparity: The Case of Cu- and Ag-Based KesteriteCompounds,” Adv. Funct. Mater., vol. 25, issue 43, pgs. 1-11 (October2015), the contents of which are incorporated by reference as if fullyset forth herein.

As highlighted above, another way to consider this S concentrationgradient amongst the layers of light absorbing materials is by the S toSe ratio. Generally, the S to Se (S:Se) ratio increases incrementally asone moves up the stack. For instance, in the present example (FIG. 7),the S:Se ratio of the first layer of light absorbing material (LightAbsorbing Material 1) is 0 since there is no S in that layer. The S:Seratio of the second layer of light absorbing material (Light AbsorbingMaterial 2) is between 0 and 1 (e.g., from about 0.5 to about 0.95, andranges therebetween) due to the inclusion of both S and Se.

As highlighted above, band gap grading can also be achieved through theuse of different light absorbing materials (with different compositions)having different band gaps. See, for example, FIG. 8. As shown in FIG.8, the first layer of light absorbing material (Light Absorbing Material1) is CZTSe and the second layer of light absorbing material (LightAbsorbing Material 2) is AZTSe. CZTSe has a band gap E_(g) of about 1.0eV and AZTSe has a band gap E_(g) of about 1.33 eV. It is notable thatin this example, neither layer contains S. That is however merely anexample, and S can be included in the first and/or second layer of lightabsorbing material (i.e., CZT(S,Se) and/or AZT(S,Se)) if so desired toincrease the band gap. As described above, CZT(S,Se) has a band gapE_(g) of from about 1.2 eV to about 1.4 eV, and ranges therebetween.AZT(S,Se) has a band gap E_(g) of from about 1.3 eV (e.g., with Se andno S, i.e., AZTSe) to about 2.01 eV (e.g., with S and no Se, i.e.,AZTS), and ranges therebetween (depending on the S:Se ratio).

Embodiments having an absorber containing more than two layers of lightabsorbing materials are also contemplated herein. See, for instance,FIG. 9. In the example shown in FIG. 9, a concentration gradient of Samongst the layers of light absorbing materials is used to create a bandgap gradient in the absorber 302. By way of example only, CZT(S,Se) isused as the light absorbing material in this instance. The Sconcentration is graded by employing a first layer of light absorbingmaterial (Light Absorbing Material 1) having Se and no S (i.e., CZTSe),then a second layer of light absorbing material (Light AbsorbingMaterial 2) having both S and Se (i.e., CZT(S,Se)), thereby increasingthe band gap through the inclusion of S. Finally, an additional lightabsorbing material is added to the stack (Additional Light AbsorbingMaterial) having S and no Se (i.e., CZTS), thereby achieving the highestband gap in the stack (e.g., a band gap E_(g) of about 1.45 eV).

Another way to characterize this configuration is through reference tothe S:Se ratio of the various layers. In the present example (FIG. 9),the first layer of light absorbing material (Light Absorbing Material 1)has an S:Se ratio of 0 (since there is Se and no S). The second layer oflight absorbing material (Light Absorbing Material 2) has an S:Se ratioof between 0 and 1 (since there is Se and no S) (e.g., from about 0.5 toabout 0.95, and ranges therebetween) due to the inclusion of both S andSe. Finally, the additional layer of light absorbing material(Additional Light Absorbing Material) has an S:Se ratio of 1 (sincethere is S and no Se).

Both concentration and composition tuning techniques can also beimplemented in the same absorber stack to tune the band gap. See, forexample, FIG. 10. For instance, building on the configuration presentedin FIG. 8 (see above) where a CZTSe bottom layer and an AZTSe top layerare employed, here an intermediate layer of CZT(S,Se) is placed inbetween the top and bottom layers. As shown in FIG. 10, by including anintermediate layer (with an intermediate band gap value, a more gradualband gap grading (as compared to that shown in FIG. 8) can be achieved.Further, in order to insure a continual increase in band gap up thestack, it may be desirable in this case to employ a high-S concentrationAZT(S,Se) as the top layer (such as AZTS with a band gap of 2.01 eV).

The above-described diffusion barrier(s) have been found to be aneffective barrier against inter-diffusion of elements between the layersof light absorbing materials in the absorber stack. See, for example,FIG. 11. As shown in FIG. 11, during the present process the diffusionbarrier (in this case graphene) effectively maintains two separatelayers of light absorbing materials (in this case CZTSe and CZT(S,Se)—asin the exemplary configuration of FIG. 7) after annealing. Thecomposition (measured in arbitrary units (a.u.) is shown as a functionof the depth in the absorber (i.e., from the conductive layer/backcontact (in this case Mo) to the top surface of the absorber).

FIG. 12 is a diagram illustrating current-voltage (I/V) characteristicsof a solar cell prepared according to the present techniques having theabsorber configuration illustrated in FIGS. 7 and 11. As shown in FIG.12, the solar cell had a saturation current (J0) of 6.233e-007 milliampsper square centimeter (mA/cm²), a shunt resistance (Rsh) of 588.8Ohms·square centimeter (Ohm·cm²), a series resistance (Rs) of 0.0252Ohm·cm², an open circuit voltage (Voc) of 0.5437 volts (V), a shortcircuit current (Jsc) of 34.0976 mA/cm², a fill factor (FF) of 77.2090percent, and an efficiency of 14.3133 percent.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A solar cell, comprising: a substrate; a layer ofa conductive material on the substrate; an absorber on the layer ofconductive material, the absorber comprising: i) a first layer of lightabsorbing material on the layer of conductive material, ii) a diffusionbarrier directly on the first layer of light absorbing material, iii) asecond layer of light absorbing material directly on the diffusionbarrier to form a stack of layers of light absorbing materials on thelayer of conductive material, wherein the second layer of lightabsorbing material has both a different elemental composition and adifferent sulfur concentration than the first layer of light absorbingmaterial such that the second layer of light absorbing material has ahigher band gap than the first layer of light absorbing material; abuffer layer on the absorber, wherein the buffer layer forms a p-njunction with the absorber; a transparent front contact on the bufferlayer; and a metal grid on the transparent front contact.
 2. The solarcell of claim 1, wherein the diffusion barrier comprises a materialselected from the group consisting of: graphene, titanium nitride,tantalum nitride, and tungsten nitride.
 3. The solar cell of claim 1,wherein the diffusion barrier comprises graphene.
 4. The solar cell ofclaim 3, wherein the diffusion barrier comprises from about 1 layer toabout 5 layers of graphene.
 5. The solar cell of claim 1, wherein thefirst layer of light absorbing material comprises copper, zinc, tin, andat least one of sulfur and selenium.
 6. The solar cell of claim 5,wherein the second layer of light absorbing material consists ofcomprises silver, zinc, tin, and at least one of sulfur and selenium. 7.The solar cell of claim 1, wherein the first layer of light absorbingmaterial comprises copper, zinc, tin, and selenium.
 8. The solar cell ofclaim 7, wherein the second layer of light absorbing material comprisescopper, zinc, tin, sulfur, and selenium.
 9. The solar cell of claim 1,wherein the absorber further comprises: v) an additional diffusionbarrier directly on the second layer of light absorbing material and vi)an additional layer of light absorbing material directly on theadditional diffusion barrier, wherein the additional layer of lightabsorbing material has a higher band gap than both the first layer oflight absorbing material and the second layer of light absorbingmaterial.
 10. The solar cell of claim 9, wherein the first layer oflight absorbing material comprises copper, zinc, tin, and selenium witha first sulfur to selenium ratio of 0, wherein the second layer of lightabsorbing material comprises copper, zinc, tin, sulfur, and seleniumwith a second sulfur to selenium ratio of between 0 and 1, and whereinthe additional layer of light absorbing material comprises copper, zinc,tin, and sulfur with a third sulfur to selenium ratio of
 1. 11. Thesolar cell of claim 1, wherein the second layer of light absorbingmaterial has a greater sulfur concentration than the first layer oflight absorbing material.
 12. The solar cell of claim 1, wherein thesubstrate comprises a material selected from the group consisting of:glass, ceramic, metal foil, and plastic.
 13. The solar cell of claim 1,wherein the conductive material is selected from the group consistingof: molybdenum, molybdenum trioxide, gold, nickel, tantalum, tungsten,aluminum, platinum, titanium nitride, silicon nitride, and combinationsthereof.
 14. A solar cell, comprising: a substrate; a layer of aconductive material on the substrate; an absorber on the layer ofconductive material, the absorber comprising: i) a first layer of lightabsorbing material on the layer of conductive material, ii) a diffusionbarrier directly on the first layer of light absorbing material, iii) asecond layer of light absorbing material directly on the diffusionbarrier, v) an additional diffusion barrier directly on the second layerof light absorbing material, and vi) an additional layer of lightabsorbing material directly on the additional diffusion barrier, to forma stack of layers of light absorbing materials on the layer ofconductive material, wherein the second layer of light absorbingmaterial has a higher band gap than the first layer of light absorbingmaterial; and wherein the additional layer of light absorbing materialhas both a different elemental composition and a different sulfurconcentration than the first layer of light absorbing material and thesecond layer of light absorbing material such that the additional layerof light absorbing material has a higher band gap than both the firstlayer of light absorbing material and the second layer of lightabsorbing material; a buffer layer on the absorber, wherein the bufferlayer forms a p-n junction with the absorber; a transparent frontcontact on the buffer layer; and a metal grid on the transparent frontcontact.
 15. The solar cell of claim 14, wherein the diffusion barriercomprises a material selected from the group consisting of: graphene,titanium nitride, tantalum nitride, and tungsten nitride.
 16. The solarcell of claim 14, wherein the diffusion barrier comprises graphene. 17.The solar cell of claim 16, wherein the diffusion barrier comprises fromabout 1 layer to about 5 layers of graphene.
 18. The solar cell of claim14, wherein the first layer of light absorbing material comprisescopper, zinc, tin, and at least one of sulfur and selenium, and whereinthe second layer of light absorbing material consists of silver, zinc,tin, and at least one of sulfur and selenium.
 19. The solar cell ofclaim 14, wherein the first layer of light absorbing material comprisescopper, zinc, tin, and selenium, and wherein the second layer of lightabsorbing material comprises copper, zinc, tin, sulfur, and selenium.20. The solar cell of claim 14, wherein the first layer of lightabsorbing material comprises copper, zinc, tin, and selenium with afirst sulfur to selenium ratio of 0, wherein the second layer of lightabsorbing material comprises copper, zinc, tin, sulfur, and seleniumwith a second sulfur to selenium ratio of between 0 and 1, and whereinthe additional layer of light absorbing material comprises copper, zinc,tin, and sulfur with a third sulfur to selenium ratio of 1.